U.S. Pat. No. 8,182,368 to Kamino et al. discloses a golf ball wherein the difference between the JIS-C hardness H4 of the core at its surface and the JIS-C hardness H3 of the core outer layer at its innermost portion is equal to or greater than 10.
U.S. Pat. No. 8,007,376 to Sullivan et al. discloses a golf ball having an inner core layer with a negative hardness gradient and an outer core layer with a positive hardness gradient. U.S. Pat. No. 7,410,429 to Bulpett et al. discloses a golf ball wherein the hardness of the inner core outer surface is the same as or lower than the hardness of the geometric center and the hardness of the outer core layer outer surface is greater than the hardness of the inner surface.
U.S. Pat. No. 6,695,718 to Nesbitt discloses a golf ball including a center core component preferably formed from a sulfur-cured polybutadiene and a core layer component preferably formed from a peroxide-cured polybutadiene and a metal salt of a fatty acid.
Despite these, and additional disclosures of golf balls having various hardness gradient properties, there remains a need for a very high positive gradient core, including a very soft, low compression inner core layer formed from an unfoamed composition. Such core would provide good durability while also contributing to spin reduction.